Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	$ 1,094,645		$ 1,219,409		$ 1,972,427	$ 2,108,601
Operation and maintenance expense	149,427		159,159		288,027	288,204
Depreciation and amortization expense	96,772		89,381		192,837	177,755
Taxes, other than income	79,093		73,007		143,581	135,780
Operating income	297,677		270,902		534,141	512,985
Other non-operating income (expense)	4,232		(2,167)		(3,491)	(4,724)
Interest charges	26,949		27,304		54,798	58,813
Income before income taxes	274,960		241,431		475,852	449,448
Income tax (benefit) expense	60,072		62,439		103,318	(43,676)
Net income	214,888	$ 157,646	178,992	$ 314,132	372,534	493,124
Capital expenditures	361,182		310,740		777,586	693,978
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	10,971,734				10,971,734		$ 10,371,147
Total assets	12,674,950				12,674,950		11,874,437
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues					(194,417)
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	1,057,889		1,199,291		1,896,724	2,060,083
Operation and maintenance expense	117,621		130,077		223,388	233,292
Depreciation and amortization expense	69,904		65,649		139,613	131,083
Taxes, other than income	71,053		64,692		127,243	119,799
Operating income	228,963		211,820		398,400	385,098
Other non-operating income (expense)	5,263		(1,521)		(1,214)	(3,443)
Interest charges	15,896		16,898		34,106	38,266
Income before income taxes	218,330		193,401		363,080	343,389
Income tax (benefit) expense	46,137		48,158		76,502	(50,953)
Net income	172,193		145,243		286,578	394,342
Capital expenditures	293,270		224,235		595,815	465,484
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	8,126,906				8,126,906		7,644,693
Total assets	11,904,290				11,904,290		11,109,128
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	135,650		120,955		270,120	247,418
Operation and maintenance expense	32,118		29,393		65,265	55,533
Depreciation and amortization expense	26,868		23,732		53,224	46,672
Taxes, other than income	8,040		8,315		16,338	15,981
Operating income	68,714		59,082		135,741	127,887
Other non-operating income (expense)	(1,031)		(646)		(2,277)	(1,281)
Interest charges	11,053		10,406		20,692	20,547
Income before income taxes	56,630		48,030		112,772	106,059
Income tax (benefit) expense	13,935		14,281		26,816	7,277
Net income	42,695		33,749		85,956	98,782
Capital expenditures	67,912		86,505		181,771	228,494
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	2,844,828				2,844,828		2,726,454
Total assets	3,071,654				3,071,654		2,963,480
Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(98,894)		(100,837)		(194,417)	(198,900)
Operation and maintenance expense	(312)		(311)		(626)	(621)
Depreciation and amortization expense	0		0		0	0
Taxes, other than income	0		0		0	0
Operating income	0		0		0	0
Other non-operating income (expense)	0		0		0	0
Interest charges	0		0		0	0
Income before income taxes	0		0		0	0
Income tax (benefit) expense	0		0		0	0
Net income	0		0		0	0
Capital expenditures	0		0		0	0
Intersegment eliminations | Intersegment eliminations
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0				0		0
Total assets	(2,300,994)				(2,300,994)		$ (2,198,171)
Reportable Subsegments
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	1,094,645		1,219,409		1,972,427	2,108,601
Reportable Subsegments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	1,057,192		1,198,309		1,895,373	2,058,762
Reportable Subsegments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	37,453		21,100		77,054	49,839
Reportable Subsegments | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0		0		0	0
Intersubsegment Eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	0		0		0	0
Intersubsegment Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	697		982		1,351	1,321
Intersubsegment Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	98,197		99,855		193,066	197,579
Intersubsegment Eliminations | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Total operating revenues	(98,894)		(100,837)		(194,417)	(198,900)
Natural Gas, US Regulated
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	471,676		626,960		813,841	993,877
Natural Gas, US Regulated | Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	570,348		727,053		1,008,080	1,190,811
Natural Gas, US Regulated | Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	(90)		433		(448)	1,345
Natural Gas, US Regulated | Intersegment eliminations
Segment Reporting Information Profit Loss [Abstract]
Cost of Revenue	$ (98,582)		$ (100,526)		$ (193,791)	$ (198,279)